The vice-president called on Moscow to urge the pro-Russian separatists to leave the buildings they are occupying and to abandon checkpoints.

The US is to provide an additional $50m (£30m) for political and economic reforms in Ukraine, including $11m to help run the presidential election due on 25 May.

In another US move, Washington is sending 600 troops to take part in Nato exercises in the three Baltic states and Poland.

A defence department spokesman said the decision was designed to show a strong commitment to the alliance as events unfolded in Ukraine.

'Connivance'

Announcing the decision to reactivate the military operation in eastern Ukraine, Mr Turchynov said in a statement: "I call on the security bodies to resume and carry out successful anti-terrorist measures aimed at defending Ukrainian citizens living in the east of Ukraine against terrorists."

Mr Rybak, whose body was found on Tuesday, was described as a local councillor for the Fatherland party in the nearby town of Horlivka. The other man killed has not yet been publicly identified.

Mr Rybak had gone missing recently and, according to police, his body was found in a river.

"These crimes are being committed with the full support and connivance of the Russian Federation," Mr Turchynov said.

Kiev's military operation to end the occupation of buildings began on 16 April but was suspended over the Easter period.

Funerals

In another incident, a Ukrainian military surveillance plane was hit by small arms fire over eastern Ukraine, the defence ministry said.

At the funerals

By Steve RosenbergBBC News, Sloviansk

In the Church of the Holy Spirit in the centre of Sloviansk, an Orthodox priest chanted prayers for the dead. The bodies of three pro-Russian activists, shot dead at a makeshift checkpoint on Easter Sunday, lay in open coffins.

When the coffins were carried out of the church, the crowd outside shouted "Glory to the Heroes of the Donbass!" over and over again - Donbass being the name for the Don River basin. Church bells rang out.

The people I've been speaking to here are convinced that it was Ukrainian ultra-nationalists who carried out Sunday's attack. One woman told me she was proud to be Ukrainian, but that instability and violence was pushing people here to want closer ties to Russia.

The aircraft, an Antonov AN-30, suffered minor damage over Sloviansk when it was targeted by automatic gunfire, according to the ministry. No-one was hurt and the plane returned safely to Kiev.

The funerals have meanwhile taken place of three men shot on Sunday during a raid on a checkpoint manned by pro-Russian separatists near Sloviansk.

The local separatists said the attack was carried out by ultra-nationalist Right Sector militants but Kiev called it a "provocation" staged by Russian special forces.

The bodies of those killed lay in open coffins at the funeral ceremony at the Church of the Holy Spirit in the centre of Sloviansk.

Photos released

Moscow and Washington are accusing each other of breaking last week's Geneva accord on resolving the Ukraine crisis, and the US is planning further sanctions should Russia fail to fulfil its Geneva commitments.

Media playback is unsupported on your device

Media captionJoe Biden: "Ukraine faces a struggle for its very future"

But Russian Prime Minister Dmitry Medvedev told the Russian parliament on Tuesday that Russia would be able to "minimise the consequences" of any further sanctions.

The 17 April Geneva accord stipulated an immediate end to violence in eastern Ukraine and called on illegal armed groups to surrender their weapons and leave official buildings.

Image copyrightUkrainian diplomats via US State DepartmentImage caption
Photos released by the Ukrainian government purport to show a soldier, circled in red, in both Kramatorsk and Sloviansk, and in a photo (centre) showing a group in the Russian Special Forces

Amid Moscow's denials of being behind the unrest in eastern Ukraine, the government in Kiev released photos, also distributed by the US State Department, it said showed Russian soldiers among militants holding official buildings in eastern Ukraine.

There was no immediate response to the pictures from the Russian government.

Ukraine has been in turmoil since last November, when Kiev was gripped by protests over whether the country should lean more towards Russia or Europe.

Also on Tuesday, Crimea's Muslim Tatars said the leader of their community, Mustafa Dzhemilev, had been banned by Russia from returning to Crimea for five years.